Tonometer

ABSTRACT

A tonometer, for determining intraocular pressure, has a body, which can be a transparent, substantially tubular body. A plunger is located within the body and a coil spring acts between the body and the plunger. A marker member is frictionally retained within the body and is displaced relative to an external scale on the body. In use, the tonometer is brought up against the eyelid of a closed eye and the body displaced relative to the head of the plunger, until the pressure is sufficient to create a pressure phosphene. The device is then removed and the displacement of the marker member, indicative of the applied pressure is read. This reading corresponds to the intraocular pressure. The device may include a return member for returning the marker member to a zero or rest position.

FIELD OF THE INVENTION

This invention relates to an apparatus for and a method of measuringintraocular pressure in the human eye. This invention more particularlyrelates to an applanation tonometer for measuring such intraocularpressure in the human eye.

BACKGROUND OF THE INVENTION

Now, it is well known that excessive internal pressure within the humaneyeball is a component of glaucoma, a disease of the eye. This diseaseaccounts for a significant percentage of all blindness. Surveys haveshown it to be present and unrecognized in a significant number ofpeople, particularly people over the age of 40 and even more so forpeople over the age 50.

Now, it is also known that where the presence of glaucoma can beidentified at an early stage, damage to the eye and subsequent blindnesscan be arrested. Appropriate medication and surgery can serve to arrestthe progress of the disease so that useful vision is retained.

In view of the fact that glaucoma is widespread, numerous proposals havebeen made for measuring the internal eyeball pressure. Many of these arecomplex precision instruments, which are expensive, and which requireelaborate clinical settings for their operation. Typically, suchinstruments apply an amount of force to the eyeball, sufficient to allowan objective measurement of specific flattening (applanation) orindenting (indentation) of the surface of the eye. The amount of forcerequired to achieve a certain applanation or indentation is correlatedwith the intraocular pressure measured internally, and usually expressedin mm of mercury.

Conventionally, the clinical instrument involved has some element whichis applied directly to the cornea of the open eye to measure theapplanation or indentation of the cornea. In view of natural humanreflexes, this requires a topical anesthetic. The equipment is complexand costly and requires a trained and sophisticated technician tooperate it. Other proposals have been made, and the following patentslist proposals known to the applicant: U.S. Pat. Nos. 1,637,421;1,661,718; 2,656,715; 5,176,139; and 5,197,473; French Patent 2,542,603;and Russian Federation Patents 2,004,187 and 457,466.

The Lipschutz U.S. Pat. No. 1,637,421 is a pressure indicator. It is notconcerned with measuring eyeball pressure, but rather it is concernedwith applying pressure to other parts of the human body. It is based onthe well known phenomenon that sensitivity to pressure of an area of thebody is an indication of disease. More particularly, it relies on thefact that the progress of the disease is related to the sensitivity ofan associated area of the body. As such, it provides a device enablingthe pressure applied to a particular area to be measured, so thispressure can be correlated with the progress of the disease. No cleardirections are given, with regard to applying this technique to thehuman eye. Measuring pressure in the human eye presents unique anddifficult problems, as compared to other parts of the anatomy. As thehuman eye is sensitive and delicate, everyone has a strong, naturalreflex to close their eyes, if any attempt is made to touch the eye.This Lipschutz patent does not address this issue.

A hardness testing device is disclosed in U.S. Pat. No. 1,661,718 whichis of marginal relevance.

An ocular tension indicator is disclosed in the Tolman U.S. Pat. No.2,656,715. However, this requires the eyeball to be contacted. It reliesupon relative axial displacement of different components of known, setweight, to determine the pressure within the eye. As such, it appears tobe a delicate, precision instrument. Since it must contact the nakedeye, it cannot be used outside of a clinical setting.

The two Fedorov U.S. Pat. Nos. 5,176,139 and 5,197,473 disclose anocular tonometer and a related method. This relies on a somewhat uniquetechnique where a ball is permitted to fall freely onto aneyelid-covered cornea. The kinetic energy of the ball deforms a cornea.The amount of the ball rebound varies depending upon the amount ofintraocular pressure and this is judged against the height of the ballrebound. This technique would appear difficult to carry out, since itdepends upon judging the height of the ball rebound.

Russian Patent 457,466 discloses an intraocular pressure transducer.This relies upon a Hall effect generator. Weights determine thepenetration force of a plunger, whose displacement is sensed by the Halleffect generator with an output proportional to the displacement.Russian Patent 2,004,187 discloses an eye tonometer having a hollowcylindrical body with tips and working end face surfaces. It is notclear how this device is intended to work. In any event, it is againintended to be applied to the naked eyeball, which again would requirethe application of a topical anesthetic in a clinical setting.

Now, one of the problems with measuring intraocular pressure is that itcan vary during the course of the day, and even from hour to hour.Accordingly, it is highly desirable to provide some simple, inexpensivetechnique for measuring this pressure. This technique should enable anordinary person to measure the intraocular pressure within their eyes,without requiring complex expensive equipment, without requiringattendance at a clinic or the like, and without requiring the time ofhighly trained clinical staff.

SUMMARY OF THE PRESENT INVENTION

In accordance with a first aspect of the present invention, there isprovided an applanation tonometer, for measuring pressure within thehuman eye, the tonometer comprising:

a main body which is generally tubular and defines a bore;

a plunger slidably mounted within the bore of the main body andincluding a head at one end located outside the bore for contacting aneyelid and the other end of the plunger being retained within the boreof the main body, wherein the head is unitary and is sufficiently largethat, in use, the head flattens an eyeball and subjects an eyeball toapplanation;

a spring biassing means acting between the plunger and the main body,biassing the head away from the main body;

a scale means provided on the main body, for providing an indication ofthe displacement of the plunger; and

a marker member frictionally retained within the bore of the body andviewable from the exterior, the marker member being displaceablerelative to the scale means so as to provide an indication relative tothe scale means of the displacement of the plunger;

wherein the scale means is the only means in the apparatus for measuringa characteristic of an eye.

Preferably, a first end of the body, adjacent the plunger, includes afirst radially inwardly extending lip means and the plunger at the otherend thereof includes a first radially outwardly extending projectionmeans, the first lip means and the first projection means beingdimensioned such as to retain the other end of the plunger within thebody.

Conveniently, the body is generally elongate and is molded from atransparent material.

The tonometer preferably includes a return member slidably mounted inthe bore and extending from a second end of the body, the return memberpermitting a user to displace the marker member. In accordance with afurther aspect of the present invention, there is provided anapplanation tonometer, for measuring pressure within a human eye, thetonometer comprising: a main body, which is generally tubular, defines abore and includes a portion bearing a scale; a plunger slidably mountedwithin the bore of the main body and including a head at one end forcontacting an eyelid and the other end of the plunger being retainedwithin the main body; spring biasing means acting between the plungerand the main body, biasing the head away from the main body, the headbeing dimensioned such that, in use, an eyeball is flattened and subjectto applanation; and a marker member frictionally retained within thebore of the body for displacement relative to the scale by the plunger,to indicate a maximum load applied to the plunger, wherein the scalebearing portion of the body is transparent, to permit the location ofthe marker relative to the scale to be viewed from the exterior; and areturn member slidably mounted in the bore and extending from a secondend of the body, the return member permitting a user to displace themarker member.

In accordance with another aspect of the present invention, there isprovided a method of diagnosing the presence of abnormal pressure withinan eyeball of a subject, the method comprising the steps of:

(1) providing a tonometer, for measuring pressure within a human eye,the tonometer comprising: a main body; a plunger slidably mountedrelative to the main body and including a head at one end for contactingan eyelid and the other end of the plunger being retained by the mainbody, the head being sufficiently large to cause, in use, flattening andapplanation of an eyeball; spring biasing means acting between theplunger and the main body, biasing the head away from the main body; anda marker member frictionally by one of the main body and the plunger fordisplacement relative thereto, to indicate a maximum load applied to theplunger; and a scale provided on said one of the main body and theplunger, for indicating the magnitude of said maximum load;

(2) ensuring that the marker member is initially located adjacent a zeroposition on the scale;

(3) locating the head of the plunger against an eyelid;

(4) displacing the body towards the eyelid, so as to apply pressurethrough the biasing means, the head of the plunger and the eyelid to theeyeball to cause applanation of the eyeball;

(5) when the subject notices a pressure phosphene, terminatingdisplacement of the body and removing the tonometer;

(6) reading the location of the marker relative to the scale on thebody, to determine the intraocular pressure.

The method can include the following additional step:

(7) subsequently returning the marker member to the zero position.

A pressure phosphene is an apparent spot or glow of light, or an arc oflight, detected by the subject.

Advantageously, the tonometer includes a return member slidably mountedwithin the bore of the body and extending through the other end of thebore, and then step (7) comprises displacing the return member todisplace the marker member to the zero position.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawing FIG. 1, which shows apreferred embodiment of a tonometer of the present invention, in sideview and in partial section, and which also shows the tonometer in use.

DESCRIPTION OF PREFERRED EMBODIMENT

An applanation tonometer in accordance with the present invention isgenerally designated by the reference 10 in the drawings. The tonometer10 has a main body or housing 12. The body 12 is essentially tubular andis adapted to be gripped and held by a user. For this purpose, it caninclude a specific section molded or shaped to ensure good gripping.

The right hand end of the body 12, as shown in the drawing, includes aninwardly turned lip 14, for retaining a plunger, as detailed below.Additionally, an outwardly extending flange 16 is provided, tofacilitate gripping of the device, and pressing of the device against ausers eye, as detailed below.

At the left hand end, as viewed in the drawings, the body 12 includes asecond inwardly extending lip 18.

On the outside of the body, there is a graduated scale 20 provided withnumerical markings, to indicate a force supplied by the device. At leastthis portion of the body 12 should be of uniform section, but it is notimportant that the whole of the body 12 be of uniform section.

The body 12 defines an internal bore 22. As noted, at least where thescale 20 is provided, the body 12 should have constant cross section, sothat at this location the bore 22 would similarly have constant crosssection. Here, a marker 24 is slidably mounted within the bore 22. Themarker 24 is formed from resilient material, and is dimensioned to be aslight interference to fit within the bore 22, so as to be frictionallyretained within the bore 22. The dimensions of the marker 24 should besuch as to securely retain it at any position within the bore 22adjacent the scale 20, while at the same time enabling it to be freelymoved by displacement of a plunger, as detailed below. In particular,the force required to overcome the frictional retention of the marker 24should not be so great as significantly to affect a force readingobtained. Further, at least the scale portion 20 of the body 12 must betransparent, or at least partially transparent, to permit the positionof the marker 24 to be seen.

Extending out from the main body 12 is a plunger or contact member 26.This plunger 26 comprises a shaft 28, an annular retaining protection30, and a head 32. The head or contact member 32 can have any desiredshape. It is preferred for it to present a flat, circular disk surface34, and the profile to the left of that, as viewed in the drawing, isnot critical.

The projection 30 and lip 14 are both generally annular. The annularprojection 30 is dimensioned so as to have a slightly greater diameterthan the internal diameter of the lip 14. The difference in the twodimensions is sufficiently small to enable the tonometer 10 to beassembled by simply pressing the projection 30 through the lip 14,displacing the lip 14 radially outwards. For this purpose, the end ofthe body 12, adjacent the lip 14 can be provided with two or moreaxially-extending slots, so that right hand end portions of the body 12,as viewed, can be displaced radially outwards. Correspondingly, theprojection 30, as shown, can have a conical surface 38 for abutting thelip 14 and a planar surface 40 for abutting the lip 14 once it'sassembled.

A helical coil spring 36 is mounted, for compression loading, around theshaft 28. One end of the spring 36 abuts the outside of the lip 14,while the other end of the spring 28 abuts one side of the head 32. Thevarious dimensions are such that, once assembled, the spring 36 is notimmediately in compression. There is a certain amount of loose play.This is taken up to bring the marker 24 adjacent the zero point on thescale 20, as detailed below. The force required to displace the marker24 is much less than any significant load applied by the spring 36.Then, as the marker 24 is displaced, the load on the head 32 is almostsolely that applied by the spring

Now, the inner end of the plunger 24, indicated at 42 is planar andadapted to abut the marker 24, to displace it, without becoming attachedto it. For displacing the marker 24 in the other direction, a returnmember 44 is provided. This return member 44 has an annular retainingprojection 46. The annular retaining projection 46 and lip 18 interact,in essentially the same manner as the lip 14 and projection 30. Again,the left hand end of the body 12, as viewed in the drawings, can beformed to facilitate engagement of the return member 44. The returnmember 44 has an abutment surface 48 for displacing the marker 24. It isotherwise mounted for free sliding movement within the body 12. It mayoptionally be provided with a spring 49, to keep it in an extendedposition.

In use, it is first ensured that the marker 24 is adjacent the zeropoint on the scale 20. If necessary, the return member 44 is displacedinto the body 12 to push the marker 24 into this position.

Then, the head 32 is brought up against the eyelid of a closed eye, thiseyelid being indicated at 50. The head 32 is applied to the upper medialaspect of the eye, away from the cornea. This is done by the subject oruser turning the eye outerwards and slightly downwards. The mostconvenient area may differ from person to person. The user just graspsthe body 12 and places the end surface 34 against the eyelid 50. Withthe surface 34 abutting the eyelid 50, the main body 12 is displacedtowards the eyelid 50, with the user's fingers pressing against theflange 16 if desired. This displacement drives the plunger 26 into thebody 12, displacing the marker member 24 along the scale 20. Thisincreases the load on the head 32 imparted by the spring 28.

This displacement continues, increasing the pressure on the head 32until the user detects, within their eye, a spot or glow of light, or anarc of light, known as a pressure phosphene. The user then stopsdisplacement of the body 12 and removes the tonometer 10 from the eye.The spring 28 will then displace the plunger 26 out of the body 12.However, the marker 24 will be frictionally retained at a location onthe scale 20. This location will indicate the load or force of which thepressure phosphene occurred. This is indicative of the intraocularpressure within the eye.

Either a direct pressure reading can be indicated on the scale 20, orthe user can be provided with a table correlating the scale reading 20with intraocular pressure. In either case, standard clinical tests couldbe carried out to correlate the intraocular pressure measured by thedevice of the present invention, i.e. the pressure at which a pressurephosphene is detected, with the actual pressure within the eye. Thiscalibration is done against Goldmann applanation tonometry, which is acorrelated and standard test. This correlation allows for variations intissue rigidity in the eyelid and sclera and for these to becompensated.

It is realised that the normal pressure for different groups of peoplemay vary. Thus, the normal for certain people may be a low reading. Forsuch people, problematic pressure could be indicated as simply anaverage pressure.

To accommodate this, the pressure scale needs, in effect, to be variedfor different types of users. This can be achieved in various ways.Where the scale on the device itself is to be correlated with a table,then it is simply a matter of providing different tables appropriate fordifferent users. If a scale is provided with indicated pressurereadings, then different springs with different spring rates could beused, effectively to vary the pressure applied for different readings onthe scale. Another option may be to vary the diameter of the head whereit contacts the eyelid, and this could possibly be used in combinationwith different springs. Normally, the head 32 would have the same sizeand profile as a Goldmann tonometer, to facilitate comparison of thetwo. Then, before a device is given to a patient or user, the patientwould be tested to determine to which category or group of people he orshe belongs. Once determined, an appropriate device can be provided, forhome testing.

If the measured pressure is outside desired limits, then the user willbe instructed either to apply medication previously provided to the userand/or to see an appropriate medical specialist for the condition to bechecked further. For example, if the user has a known condition ofexcess intraocular pressure, for intraocular pressure within a certainexcess pressure range, the user could simply be instructed to takepreviously prescribed medication. However, if pressure even higher thanthis set pressure is detected, then the user could be instructed to seehis or her doctor.

As noted above, to return the marker 24 to the zero position, forfurther use, for example on the other eye, the return member 44 issimply grasped and pressed against the marker 24 to displace it to thezero position. Which end of the marker 24 represents zero is purelyarbitrary, and can be indicated in instructional material provided withthe device.

The tonometer of the present invention has the great advantage that itis simple and robust. It can be produced at very little cost. Itsgreatest advantage is that it can be used by an ordinary person orpatient, and does not require highly trained clinical personal. Moreimportantly, it can be used in any setting, and does not requireattendance at a clinic, hospital or the like.

While a preferred embodiment of the invention has been described, itwill be appreciated that numerous variations are possible, within thespirit of the present invention. For example, while the return member 44has been described as separate from the marker 24, it could be attachedthereto where the spring 49 is omitted. This could be achieved byproviding the marker 24 as a disk of resilient material having a centralbore, with the disk of material mounted on an end of the return member44, provided with a collar to retain the marker on the return member.The return member could then simply be molded in plastic so as to be afree sliding fit, and be carried with the marker as it is displaced.

It is preferred for the main body 12, the plunger 26 and the returnmember 44 all to be molded from a plastic material. As noted above, thescale portion 20 at least of the body 12 should be transparent, so thatthe marker 24 can be viewed through the body 12 against the scale 20.Conveniently, the whole body 12 is simply formed from transparentplastic material, of the type commonly used for syringes and the like.

The marker member could be carried by either one of the main body 12 andthe plunger 26, with the scale provided by whichever element carries themarker member.

I claim:
 1. An applanation tonometer, for measuring pressure within thehuman eye, the tonometer comprising:a main body which is generallytubular and defines a bore; a plunger slidably mounted within the boreof the main body and including a head at one end located outside thebore for contacting an eyelid and the other end of the plunger beingretained within the bore of the main body, wherein the head is unitaryand is sufficiently large that, in use, the head flattens an eyeball andsubjects an eyeball to applanation; a spring biassing means actingbetween the plunger and the main body, biassing the head away from themain body; a scale means provided on the main body, for providing anindication of the displacement of the plunger; and a marker memberfrictionally retained within the bore of the body and viewable from theexterior, the marker member being displaceable relative to the scalemeans so as to provide an indication relative to the scale means of thedisplacement of the plunger; wherein the scale means is the only meansin the apparatus for measuring a characteristic of an eye.
 2. Atonometer as claimed in claim 1, which includes a return member slidablymounted in the bore and extending from a second end of the body, thereturn member permitting a user to displace the marker member.
 3. Atonometer as claimed in claim 1, wherein the marker member is secured tothe return member for movement therewith.
 4. A tonometer as claimed inclaim 1, wherein the body at a first end, adjacent the plunger, includesa first radially inwardly extending lip means and the plunger at theother end thereof includes a first radially outwardly extendingprojection means, the first lip means and the first projection meansbeing dimensioned such as to retain the other end of the plunger withinthe body.
 5. A tonometer as claimed in claim 4, wherein the firstprojection means comprises an annular projection having a generallyconical surface, reducing in diameter in a direction away from the oneend of the plunger, and a planar surface, the conical surface serving todisplace the first retaining lip means radially outwards to permitinsertion of the plunger during assembly of the tonometer and the planarsurface being dimensioned to abut the first retaining lip means toretain the one end of the plunger within the body.
 6. A tonometer asclaimed in claim 5 wherein the spring biasing means comprises a helicalcoil spring provided around the plunger and abutting the body and theone end of the plunger.
 7. A tonometer as claimed in claim 6, whereinthe body is generally elongate and is molded from a transparentmaterial.
 8. An applanation tonometer, for measuring pressure within ahuman eye, the tonometer comprising: a main body, which is generallytubular, defines a bore and includes a portion bearing a scale; aplunger slidably mounted within the bore of the main body and includinga head at one end for contacting an eyelid and the other end of theplunger being retained within the main body; spring biasing means actingbetween the plunger and the main body, biasing the head away from themain body, the head being sufficiently large that, in use, an eyeball isflattened and subject to applanation; and a marker member frictionallyretained within the bore of the body for displacement relative to thescale by the plunger, to indicate a maximum load applied to the plunger,wherein the scale bearing portion of the body is transparent to permitthe location of the marker relative to the scale to be viewed from theexterior; and a return member slidably mounted in the bore and extendingfrom a second end of the body, the return member permitting a user todisplace the marker member.
 9. A tonometer as claimed in claim 8,wherein the second end of the body includes a second radially extendinglip means and the return member includes one end, located within thebody and including a second radially outwardly extending annularprojection means, the second lip means and the second annular projectionmeans being adapted to retain the return member within the body.
 10. Atonometer as claimed in claim 9, wherein the marker member is secured tothe return member for movement therewith.
 11. A tonometer as claimed inclaim 1, 8 or 10, wherein the marker member is formed from resilientpolymeric material, dimensioned to be frictionally retained by the boreof the body, but being capable of being displaced by the plunger withoutsignificantly affecting the load applied to the plunger.
 12. A method ofdiagnosing the presence of abnormal pressure within an eyeball of asubject, the method comprising the steps of:(1) providing a tonometer,for measuring pressure within a human eye, the tonometer comprising: amain body; a plunger slidably mounted relative to the main body andincluding a head at one end for contacting an eyelid and the other endof the plunger being retained by the main body, the head beingsufficiently large to cause, in use, flattening and applanation of aneyeball; spring biasing means acting between the plunger and the mainbody, biasing the head away from the main body; and a marker memberfrictionally by one of the main body and the plunger for displacementrelative thereto, to indicate a maximum load applied to the plunger; anda scale provided on said one of the main body and the plunger, forindicating the magnitude of said maximum load; (2) ensuring that themarker member is initially located adjacent a zero position on thescale; (3) locating the head of the plunger against an eyelid; (4)displacing the body towards the eyelid, so as to apply pressure throughthe biasing means, the head of the plunger and the eyelid to the eyeballto cause applanation of the eyeball; (5) when the subject notices apressure phosphene, terminating displacement of the body and removingthe tonometer; (6) reading the location of the marker relative to thescale on the body, to determine the intraocular pressure.
 13. A methodas claimed in claim 12, which includes the following additional step:(7)subsequently returning the marker member to the zero position.
 14. Amethod as claimed in claim 13, wherein the tonometer includes a returnmember slidably mounted within the bore of the body and extendingthrough the other end of the bore, wherein step (7) comprises displacingthe return member to displace the marker member to the zero position.15. A method as claimed in claim 12, 13, or 14 which comprises applyingthe head of the tonometer to an upper medial aspect of the eye, awayfrom the cornea.